1. Field of the Invention
The present invention relates to a joint assemblies for use in drive trains and, more particularly, to drive trains within downhole rotational tools.
2. Description of Related Art
Subterranean fluids, such as oil, gas and water, are often pumped or "lifted" from wellbores by the operation of downhole pumps, such as by electric submergible pumping systems (ESP's). ESP's typically use an elongated electric motor installed within the wellbore to rotate a multistaged centrifugal pump. While centrifugal pumps are widely used for the recovery of subterranean fluids, such centrifugal pumps have difficulty in lifting viscous fluids, such as from Southern Calif., and fluids with relatively high concentrations of sand and other abrasive materials, such as from the tar sands area of Alberta, Canada. Thus, there is a need for a downhole pump that can lift such fluids.
Canadian Patent 924,181 discloses a solution to the problem using an ESP to recover viscous fluids and fluids with relatively high concentrations of sand with an ESP, and the solution consists of connecting a Moineau pump or a progressive cavity pump (PCP) though a gear reduction system to an ESP's electric motor. Progressive cavity pumps have been proven efficient in recovering relatively viscous fluids and fluids with relatively high concentrations of sand. Examples of progressive cavity pumps are disclosed in U.S. Pat. No. 3,627,453; 4,080,115; and 5,048,622. One major hindrance to the successful operation of such a pumping system has been that the progressive cavity pump inherently causes oscillations and gyrations that propagate through the pumping system's drive train. These oscillations and gyrations are hereinafter referred to as "non-coaxial" forces, and have been found to be of sufficient magnitude to damage the bearings in the ESP's electric motor and thus cause the ESP to prematurely fail. The failed ESP must then be removed from the wellbore, which causes the operator to suffer loss of production, loss of revenue and additional repair costs.
The non-coaxial forces are caused by the rotor in the progressive cavity pump being in the shape of a helix contained within a cavity or a flexible lining within a housing, referred to as the stator. The rotor rolls with respect to the stator so that the rotor and stator form a series of sealed cavities which are approximately 180 degrees apart. As one cavity increases in volume, its counterpart cavity decreases in volume, at exactly the same rate. The driving motion of the rotor is quite complex in that it is simultaneously rotating and moving transversely with respect to the stationary stator's liner. The rotation of the true center of the rotor traces a circle progressing the opposite direction to the rotation of the rotor, but with the same speed. Thus, the rotor driving motion is simultaneously a rotation, an oscillation, and a reverse orbit. It is a combination of these motions acting at the point of interconnection of the rotor to the electric motor that cause the undesired non-coaxial forces to be destructively transmitted to the bearings in the ESP's electric motor.
U.S. Pat. No. 5,408,622 discloses a dual universal joint mechanism for interconnecting a progressive cavity motor with a drill bit. The dual universal joint mechanism disclosed in U.S. Pat. No. '622 tries to solve the same problem of non-coaxial forces as above described by permitting limited non-parallel axial deflection or "buckling" of the drive train in response to such forces about two conventional dual-yoked universal joints. However, it has been found that this "buckling" causes the overall length of the assembly to be reduced thereby requiring an additional slip joint connector. Further, once the universal joint has been deflected or "buckled out of parallel axial alignment", and compression forces are added, there is no way for the universal joint to un-buckle and be realigned so that it can move in response to noncoaxial forces.
There is a need for a joint assembly for use in the drive train of a downhole tool that can efficiently and reliably reduce or eliminate such non-coaxial forces so that a progressive cavity pump can be successfully connected to and used with an ESP's electric motor.